Radiation from the sun is mainly in the ‘short-wave’ range from 300 to 4000nm (4μm) that includes the visible and ultraviolet. A proportion of this radiation is absorbed by clouds, aerosols and molecules in the atmosphere, which warms up and radiates ‘long-wave’ radiation. This is far infrared thermal energy (FIR) at wavelengths from 4.5μm to beyond 40μm. Both the shortwave and long-wave radiation reach the Earth, where some is reflected and the remainder warms up the surface. The Earth radiates long-wave thermal energy back to the sky.

The relationship of incoming and outgoing short-wave and long-wave radiation is the ‘Energy Balance’. Short-wave radiation is measured by a pyranometer and the long-wave radiation by a pyrgeometer. The CGR3 is especially designed so that two instruments can be mounted base to base and fitted with the optional mounting rod. This configuration forms an albedometer and allows the sensors to monitor both the incoming radiation from the sky and the reflected outgoing radiation from the Earth's surface. An integrated 10K thermistor, or PT100, temperature sensor in the CGR allows readings to be compensated for heating offset, although this is irrelevant for net radiation calculations as both pyrgeometers should be the same temperature.

Specification

For the best accuracy the CGR 3, should be shielded from the direct short-wave solar radiation which may heat up the pyrgeometer window and cause significant thermal offsets. The direct solar radiation can be intercepted by a small disk or sphere and the shadow must cover the pyrgeometer dome completely. However, to follow the sun's apparent motion, a Sun tracker, such as the 2AP or Solys 2, can be used to track the sun accurately under all weather conditions. Alternatively, a static shadow ring, CM 121 can be used to intercept the direct solar radiation; but it is less accurate and will require periodic manual adjustment. At times the shadow ring also intercepts a proportion of the diffuse sky radiation. Therefore, corrections for this to the recorded data would be necessary. Further information on these items can be found in our Suntrackers section of the website. The CGR 4 does not require shading from direct short-wave solar radiation because the dome-heating effect, when suitably ventilated, is negligible due to the unique construction of the pyrgeometer.